All cells that are making proteins. Obviously this will be all cells in the plant at some stage of development. Therefore the ALS gene has a promoter sequence that allows the gene to be turned on in all types of cells. When the ALS gene is turned on, transcription can take place. Like most chemistry in the cell, enzymes will be needed for gene expression. RNA polymerase is the transcription enzyme Fig. Figure 9a. The promoter has DNA sequences that signal the cell when to turn on the gene and where to start reading the gene information.
Figure 9b. RNA Polymerase binds to the promoter to initiate transcription. Image by D. Figure 1 shows how this occurs. Figure 1. Overview of Transcription. Triplets are groups of three successive nucleotide bases in DNA. Codons are complementary groups of bases in mRNA. Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in Figure 2. Figure 2. But how then do we explain the EM data that most of the observed transcripts in Drosophila nuclei appear uncleaved upon termination Osheim et al.
One possibility is that many poly A sites are sufficient on their own to induce termination by allosteric modifications, thereby bypassing the requirement for exonuclease degradation of the downstream RNA. Indeed, the finding that Rat1 seems to function as an integral component of the cleavage complex Luo et al. Once the EC is in the termination-ready state, an additional signal might be required to stimulate transcript release Fig.
Evidence from Luo et al. Significantly, if Pcf11 functions in cleavage and termination are indeed inseparable, this removes another apparent argument against the torpedo model.
Thus, a parsimonious view of available data suggests that exonuclease activity is indeed necessary, but not sufficient, for termination. Although it is not known whether TTF2 functions during interphase transcription, TTF2, or a similar factor, might be recruited to stimulate template release. Indeed, if Rat1 or an associated protein specifically recruits a TTF2-like helicase, then this provides an explanation why Rat1 but not Xrn1 which would be unable to recruit the helicase is able to function in termination.
Such cooperative termination activity is well documented in bacteria and vaccinia virus RNA polymerase Deng and Shuman ; Nudler and Gottesman But the finding that something in addition is required is important and opens additional possibilities.
For example, neither model—the torpedo nor the allosteric—in its simplest form is sufficient to explain what signals provoke Pol II to release the template.
While the torpedo model has been considerably strengthened, new integrated models have now been proposed. With these important new insights in hand, it will be interesting to see what new discoveries, such as the identification of an essential termination helicase, will be forthcoming.
We thank I. Boluk for assistance in the preparation of the manuscript. Work in our laboratory is supported by funds from the National Institutes of Health.
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